The increasing demand for high data rates in cellular networks requires new approaches to meet this expectation. A challenging question for operators is how to evolve their existing cellular networks so as to meet the requirement for higher data rates. In this respect, a number of approaches are possible: i) increase the density of their existing base stations, ii) increase the cooperation between base stations, and/or iii) deploy smaller base stations (low power nodes, or LPNs) in areas where high data rates are needed within a base stations grid.
The option of deploying smaller base stations is in general referred to in the related literature as a “Heterogeneous Network”, or “Heterogeneous Deployment” and the layer consisting of smaller base stations is termed a “micro”, or “pico” layer. The larger base stations are then referred to in this context as “macro” base stations.
Building a denser macro base station grid, while simultaneously enhancing the cooperation between macro base stations (hence either using options i) or ii) above) is a solution that meets the requirement for higher data rates; however such an approach may not necessarily be a cost-efficient option, due to the costs and delays associated with the installation of macro base stations, especially in urban areas where these costs may be significant.
FIG. 1 shows the basic principle of heterogeneous deployments. Large macro cells 10, which are geographic areas nominally served by a base station, are generally able to provide coverage to a larger service area. However, the addition of smaller micro/pico cells 11 can improve network capacity in certain regions of those macro cells. Micro/pico cells are essentially subcells of a macro cell, and are served by low power, short range nodes, such as micro/pico base stations, using frequencies allocated by the macro base station. Allocation of resources between the macro and micro/pico cells can be semi-static, dynamic or shared across the macro-micro/pico layers.
One of the main objectives of micro/pico layers is to offload as many users as possible from the macro layers to the micro/pico layers. In an ideal scenario, this may enable users to experience higher data rates in both the macro and micro/pico layers.
In this respect, several techniques have been discussed and proposed within 3GPP. One proposal is to extend the range of small cells by using cell specific cell selection offsets. A cell selection offset is an additional power margin for a cell that must be overcome before a handover to the cell will occur. Setting the cell selection offset for a particular microcell to a negative value can therefore increase the probability of a handover occurring to the microcell, thereby extending the range of the microcell. Another proposed approach is to increase the transmission power of low power nodes and simultaneously setting appropriately the uplink (UL) power control target P0 for the users connected to low power nodes.
The solution of deploying small base stations within the already existing macro layer grid is an appealing option, since these smaller base stations are anticipated to be more cost-efficient than macro base stations, and their deployment time is expected to be shorter as well. Even so, there will be scenarios in which deployment of pico- or macro-base stations and their associated backhaul costs may be prohibitive. In such scenarios, the use of relay nodes that employ in-band backhaul communications may provide a viable option that provides pico cell type coverage either indoors or outdoors and mitigates the cost and effort of deploying land-line backhaul to all the pico base stations.
One of the issues with heterogeneous networks is that small base stations, even if they are easier to deploy and operate than macro base stations, still cannot be deployed everywhere, since there are restrictions on where to place them. Furthermore, often the placement of small base stations or LPNs results in insufficient coverage for all of the users targeted to be served. Hence, even after the addition of small base stations around them, there still exists the possibility of users being in coverage holes of the macro layer, and as such they may not necessarily benefit from this addition of small base stations, relays, or low power nodes. This can happen due to an obstacle, such as a wall, or similar barrier being between the low power node and the user in the macro layer coverage hole.
Moreover, such a situation like the one described above might occur even in the case of significant obstacles between the LPNs and certain users. Due to the much higher power transmitted by the macro base stations, low power nodes do not always succeed in absorbing many users. For example, there might be cases such as the macro layer not providing good coverage to a certain area, and thus users in this area could still connect to the macro base station rather than to the low power node around.
One way to extend the coverage of macro base stations without adding a micro/pico layer is to deploy remote radio heads within the macro cell. Referring to FIG. 2, a remote radio head (RRH) 30 can be used to provide an intermediate node between a user equipment unit (UE) 40 and a base station 20, referred to in this context as a “donor”, “serving” or “anchor” base station, as it is providing resources to the RRH 30. Communications between the RRH 30 and the UE 40 is performed using the Uu interface, which is the same interface that the UE 40 normally uses to communicate directly to the base station 20. Thus, from the standpoint of the UE 40, there is no difference in the protocol used when communicating with a relay node.
Communications between the RRH 30 and the donor base station 20, referred to as “backhaul” communications, are performed using the Un interface on both the uplink (RRH to base station) direction and the downlink (base station to RRH) direction.